Farm management factors associated with bulk tank somatic cell count in Irish dairy herds

Data collection

Data were obtained on annual milk supply for all farmers supplying to one of the major milk processors in Ireland, the milk processor supplied data on milk volume and bulk tank SCC on a collection basis for the years 2000 through to 2007. Milk was collected from the farms at a frequency range of one to four days. The SCC of the bulk tank was taken at almost every milk collection. Annual herd milk supply for the calendar year of 2004 was divided into strata in increments of 10,000 litres with herds supplying yields at either end of the supply distribution being merged due to small strata sizes. A total of 450 herds were randomly chosen, with the percentage selected from each stratum being weighted by the frequency of herds within strata relative to the sample population. These farms were invited to participate in a questionnaire survey, 400 of them decided to take part.

Two questionnaires were administered to each farm, one of which was undertaken during the period April to July, 2006 and the second during the period December 2006 to March 2007. In the first (summer) questionnaire, there were 70 questions relating to factors such as pre- and post-milking practices, milking machines, cleanliness of the facilities, and the practice of milk recording. The second (winter) questionnaire had 30 questions relating mainly to factors such as housing dates, housing type, cleanliness of shed and frequency of cleaning. The questions in the surveys required objective measurements and factual responses from the farmer as well as subjective measures. Bulk tank milk samples were taken during the summer visit. The sample was taken from the bulk tank after the milk was agitated, using individual sterile sample bottles. 10 μl of each sample was inoculated onto blood agar plates (base no. 2; MERCK product. Manufactured in Merck KGaA 64271 Darmstadt. Germany) and incubated at 37°C overnight (16-18 h). Bacteria were identified visually from the plates after incubation, by an experienced laboratory person.

Each participating farm was visited on two occasions (summer, winter), and the questionnaires were completed during a face-to-face interview with the farmer at each visit. The questionnaires were developed following detailed discussion with specialists working in the milk quality area and examination of the main factors known to affect SCC, such as milking parlour, milking practices, housing, and hygiene. Three people were involved in the administration of the survey. Prior to the start of the study, these people standardised their approach to the scoring of farm hygiene, which was based on a visual assessment of farm yards and parlours. In addition, a scoring system for cow cleanliness was devised, based on a random sample of ten cows from each herd. Each cow was given a composite score of one (clean) to four (very dirty) based on the component score of the udder, tail and legs. An overall (herd) cow cleanliness score was calculated by combining the individual cow scores. Farms were divided into five regions based on geographical location. Farm visits within each region were alternated across time to minimise any potential temporal by spatial bias. A paper copy of each questionnaire was completed on-farm, then subsequently entered into Microsoft Excel, where the data was managed for ease of analysis. The milk processing data was supplied in electronic form and managed using Microsoft Excel. The two sets of data were combined using SAS.

Statistical analysis

Bulk tank SCC data for all milk collections in the 365 days prior to the first visit to a specific farm were obtained and the average of the natural logarithm of SCC calculated; this variable will be referred to as somatic cell score (SCS). A strong correlation (0.97) existed between mean SCS in the 365 days prior to the first farm visit and mean SCS 365 days post the first farm visit. All analyses were undertaken using linear models in PROC GLM (SAS, 2006) with SCS included as the dependent variable. Geographical location of the farm region was included in all models as a confounding effect. Initially, a series of regression models were constructed for each independent variable together with the confounding factor. Factors that were associated with SCS at a significance level of P < 0.30, were retained for further analysis. Multiple regression models were developed using stepwise regression on the variables that fulfilled the initial selection criteria. Separate multiple regression models were generated using the questions from the summer questionnaire, winter questionnaire and from both. Statistical significance is defined as P < 0.05 for all final multi regression models. Residual diagnostics did not indicate any concern for departures from the statistical assumptions of constant variability and normality.

Herd management

The lower SCS observed in herds that practice milk recording is possibly due to increased farmer knowledge on individual cows and its importance as a factor associated with SCC was substantiated by its persistence in the multiple regression model. Hutton et al. [13] also reported that farmers that were more aware of the mastitis status of the herd had lower SCC.

The beneficial association between the use of dry cow therapy and lower SCC is probably due to minimising the carry over effect of subclinical mastitis across lactations [16]. Smith et al. [24] also showed that dry cow therapy reduced the rate of streptococcal infections during the early dry period but had no effect during the prepartum period. Wenz et al. [25] showed a trend between the use of dry cow therapy and low bulk tank SCC.

Teat preparation and hygiene

In agreement with most previous studies [3, 7] clean farms, houses and milking parlours were strongly associated with lower SCS. The importance of cleanliness and hygiene was substantiated by these factors remaining in the multiple regression model. Nonetheless, no significant association was observed in the present study between cow cleanliness and SCS, which disagreed with Reneau et al. [21] who reported lower SCC in cleaner cows in The Netherlands. Schreiner and Ruegg. [23] also reported an increase in SCS and prevalence of intramammary environmental pathogens as udder hygiene score increased within a scale of one to four, four indicating dirty cows. The cleaner the roadway and holding yard; the less chance of dirt splashing on the cow's udder both before and after milking which may reduce the exposure of the teat ends to manure. Schreiner and Ruegg [23] reported that the primary sources of exposure for environmental mastitis pathogens to the cow are the presence of moisture, mud, and manure. The more sanitised the machine, the fewer bacteria transmitted to the first line of cows from the last line of cows in the previous milking. This is increasingly important in herds with high milk SCC and mastitic cows at the end of milking. A higher frequency of passageway cleaning and cubicle cleaning and also specific bedding material types were associated with bulk tank SCC. In agreement, Chassagne et al. [7] showed that cleaner dry cow sheds were observed more frequently in the lower SCC category. Also, Barkema et al. [3] reported that the cleanliness of cubicles and a greater frequency of cubicle cleaning were associated with lower bulk milk SCC. Wenz et al. [25] documented that both the bedding material and the housing facility for cows were associated with bulk tank SCC.

Teat preparation is well researched for its association with SCC and intramammary infection rate but the results differ slightly, with the current study finding no association between pre-milking teat preparation and SCS, which is in agreement with Hutton et al. [13] who reported that there was no significant difference in numbers of herds using teat preparations in the study between high and low SCC groups. However, in contrast it has also been shown that the utilisation of pre-milking teat preparation compared with no teat preparation is significantly associated with lower bulk tank SCC [12], reduced presence of bacteria [19] and reduced incidence of new intramammary infections [10, 18]. Goldberg et al. [11] concluded that insufficient hygiene prior to milking may repress the effect of improved management practices. The difference in results between the current study and previous studies may be due to cows in the present study being milked while at pasture and therefore being less dirty and under less pathogenic load, with the subsequent effect of reducing any potential benefits of teat preparation as may be observed in confined cows. Barkema et al. [3] found the use of teat disinfection to have a reducing effect on bulk milk SCC and Chassagne et al. [7] showed that teat spraying was more predominant in the low SCC group, both of which support the current study.

Milking parlour

This study showed a difference between recorder plants and direct pipelines with regard to SCS, the explanation of which requires additional information on the parlour design, as it is unknown if the vacuum was affected or the milk line height was different on these farms. The farms with automatic cluster removers had lower SCS, potentially due to consistent cluster removal at a specific milk yield and less chance of over milking. Natzke et al. [17] showed that the increase in new infections from over milking is due to an increase in the number of quarters infected in an already infected cow rather than the number of newly infected cows increasing. Hutton et al. [13] reported that cluster removers were less frequent on high SCC herds than low SCC herds. Wenz et al. [25] also documented that the use of automatic cluster removers was associated with lower bulk tank SCC. In agreement with the present study, Hutton et al. [13] also reported that milking clinically infected cows last was more common in low SCC herds.